I’m finishing the build on the charger. As you may know, the Fenix is great watch but fails in the ultra-track mode to record GPS accurately. Fenix specs 16 hours battery life in GPS mode and I don’t know if that includes Bluetooth and GlONASS on, barely enough or not for Miwok 100K, for example. Fenix 3 has an 80 mAH battery, easily recharged several times from the small, 600 mAH 14430 Li ion battery you see in the photo.
A feature of the F3 and its firmware is to allow charging during an activity, something previously not workable on other Garmin GPS watches.
The charging cradle is extremely light and not noticeable on the wrist. You could, of course, just plug a portable, single Li 18650 charging stick into the cradle with a micro USB cable as DCR shows here.
But then you have to deal with a flopping wire, which I drives me nuts and gets snagged on anything. Placement of the components as shown below is a bit of a fail because the left side buttons are not accessible. I’m thinking about putting the battery and DC to DC converter into a small, zippered wrist pouch versus a velcro’d wrist band formerly used to hold a compass. In that way, the buttons are accessible while charging. I will probably hack directly into the cradle to bypass the micro USB connection, which sticks out of the side of the cradle.
I discovered a better battery for this project. Recall that the F3 has a 300 mAH battery. I discovered that Lithium Polymer batteries (LiPo) come in literally all sizes and can be very inexpensive because they are used in various motorized toys, particularly helicopters.